An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. A printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector including an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the chamber in order to propel a droplet out of the nozzle, or a piezoelectric device that changes the wall geometry of the ink pressurization chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other print medium (sometimes generically referred to as recording medium or paper herein) in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the print medium is moved relative to the printhead.
Motion of the print medium relative to the printhead can consist of keeping the printhead stationary and advancing the print medium past the printhead while the drops are ejected. This architecture is appropriate if the nozzle array on the printhead can address the entire region of interest across the width of the print medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the print medium and the printhead is mounted on a carriage. In a carriage printer, the print medium is advanced a given distance along a print medium advance direction and then stopped. While the print medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the print medium advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the print medium, the print medium is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
Inkjet ink includes a variety of volatile and nonvolatile components including pigments or dyes, humectants, image durability enhancers, and carriers or solvents. A key consideration in ink formulation and ink delivery is the ability to produce high quality images on the print medium. Image quality can be degraded if air bubbles block the small ink passageways from the ink supply to the array of drop ejectors. Such air bubbles can cause ejected drops to be misdirected from their intended flight paths, or to have a smaller drop volume than intended, or to fail to eject. Air bubbles can arise from a variety of sources. Air that enters the ink supply through a non-airtight enclosure can be dissolved in the ink, and subsequently be exsolved (i.e. come out of solution) from the ink in the printhead at an elevated operating temperature, for example. Air can also be ingested through the printhead nozzles. For a printhead having replaceable ink supplies, such as ink tanks, air can also enter the printhead when an ink tank is changed.
In a conventional inkjet printer, a part of the printhead maintenance station is a cap that is connected to a suction pump, such as a peristaltic or tube pump. The cap surrounds the printhead nozzle face during periods of nonprinting in order to inhibit evaporation of the volatile components of the ink. Periodically, the suction pump is activated to prime the printhead, removing some ink and unwanted air bubbles from the nozzles. The pump can be powered by a dedicated motor or by a motor, such as the media advance motor, that has other functions as well. A dedicated motor results in additional cost and takes up additional space in the printer. Prior art pumps driven from the media advance motor, such as those described in U.S. Pat. No. 7,988,255 and U.S. Pat. No. 6,793,316, are configured such that a gear train with a fairly large number of gears is needed for power transmission. Such a gear train can cause additional noise during operation, and requires additional drive power from the motor in order to turn the gears. In addition, it can take ten seconds or more to generate sufficient suction to prime a printhead using a tube pump. Printing is delayed until priming is completed.
U.S. Pat. No. 5,534,896 discloses a tubeless printhead priming cap having a rolling diaphragm defining a chamber with the diaphragm being reciprocated by a spring-returned lever having a piston on one end. After the piston decreases the volume of the chamber, the priming cap is brought into sealing engagement with the printhead. A subsequent down-stroke of the piston causes the volume of the chamber to expand, thereby producing a vacuum within the chamber for priming the printhead. To empty the chamber, the cap is rotated to an ink blotter for removing accumulated ink. Such a removal mechanism for accumulated ink adds undesirable complexity to the priming system.
Consequently, a need exists for an inkjet printer cap and pump having low cost, low operational noise, rapid generation of suction and a simple way of removing waste ink that has accumulated in the cap.